Various types of solid state imaging devices have been developed, which primarily include charge-coupled devices (CCDs) and complementary metal oxide semiconductor (CMOS) image sensor devices, as well as hybrid image sensors that based on a combination of CCD and CMOS image sensor designs. In general, CCD and CMOS solid state imaging sensors CCD image sensors operate based on the “photoelectric effect”, which occurs when silicon is exposed to light. In particular, CCD and CMOS image sensors include pixel arrays where each unit pixel includes a light receiving region including one or more photodetector elements (such as photodiodes) formed in an active silicon region of the pixel. When the light receiving region is exposed to light, photons in the visible and near-IR (infra red) light spectrums have sufficient energy to break covalent bonds in the silicon, thereby releasing electrons from the valence band into the conduction band. The amount of electrons generated is proportional to the light intensity. The photon-generated charges are accumulated by the photodetector elements in the pixel array, and then detected and processed to generate a digital image.
Historically, analog CCD image sensors have dominated the market for solid-state imaging applications due to various advantages afforded by CCD image sensors, including superior dynamic range, low FPN (fixed pattern noise) and high sensitivity to light. Advances in CMOS technologies, however, have led to the development of improved CMOS image sensor designs, thereby allowing CMOS solid state image sensors to displace CCDs in various solid state imaging applications. Solid state CMOS image sensors afford various advantages including, for example, low cost fabrication, low power consumption with a single voltage power supply, system-on-chip integration, high-speed operation (e.g., capturing sequential images at high frame rates), highly-integrated pixel arrays, on-chip image processing systems, random access to unit pixels, etc. In contrast, CCD image sensor devices are expensive to fabricate, typically require 2, 3 or more supply voltages at different clock speeds with significantly higher power consumption, and do not allow random access to unit pixels.
Conventional solid state CMOS image sensors, however, can suffer from low sensitivity and various sources of noise resulting in degraded performance. For example, some conventional CMOS image sensors are highly susceptible to noise such as dark current. As is known in the art, dark current is thermally generated charge that is produced in the absence of illumination and accumulated along with photon-generated charges. Dark current is typically generated as a result of surface damage (e.g., dangling silicon bonds) to the active silicon regions of the unit pixel, such as the photodiode region, resulting from manufacturing processes such as gate and spacer etching steps. In addition, dark current can be generated as a result of damage to the silicon at the interface between an isolation region and the active silicon region. In general, the amount of dark current produced is a function of temperature and time, and the amount of dark current produced can vary significantly from pixel to pixel depending upon operating conditions. As a result, dark current can result in reduced pixel sensitivity and lower the dynamic range of the image sensor device.
Moreover, CMOS image sensors can suffer from a phenomenon known as image lag. As is known in the art, image lag can result from an incomplete pixel reset, where the reset voltage of a photodiode or sense node of a pixel varies from a desired reference voltage reset level at the beginning of the reset operation. Moreover, image lag can result from an incomplete charge transfer from a photodiode to a sensing node of a given pixel. The ability of a CMOS image sensor to completely transfer charge between two regions depends on the electric field strength between the regions. In this regard, as CMOS image sensor devices are designed to operate with lower supply voltages to meet requirements for decreased power consumption, the ability to minimize image lag (due to incomplete charge transfer and reset) becomes more problematic.